Nitrates are major components in fertilizers, explosives and by-products of certain detonated explosives, and may enter discharge waters from the mining and agricultural industries. Nitrates promote the growth of algae and other plant life in slow-moving streams, rivers and lakes resulting in increased turbidity and oxygen depletion, eventually destroying life in such waterways. Large quantities of nitrates in drinking water are harmful to animals and humans as a result of conversion to nitrites by gastrointestinal bacteria since hemoglobin reacts with nitrites in place of oxygen, thereby causing respiratory failure. Methods for removing nitrates from solutions include ion exchange, reverse osmosis and biological methods, which are expensive and nondestructive, thereby generating wastes that must be treated.
Determination of nitrate inn concentration in water is important in environmental and marine chemistry. Nitrate levels in soil are also an important parameter for agriculture. Nitrite ions generated by reduction of nitrate ions may be detected by spectrophotometric measurement. Such reduction has been achieved by passing sample solutions through a copperized cadmium column; however, the eluted solutions contain cadmium and copper which must be eliminated before discharge. An ammonium chloride and disodium ethylenediaminetetraacetic acid buffer having a pH of about 8.5 has been used as a carrier solution for sample solutions. Hydrazine has also been utilized in the reduction step. Nitrite in the original sample may be determined by removing the cadmium column.
Photolysis of nitrates to nitrites or ammonia is known. Water denitrification by stopping the reduction of nitrate at nitrite, lowering the pH of the water, and air sparging the unstable nitrite from of the water has been described. Nitrate photoreduction in sample solutions containing humic acid or hydrazine at a pH of 10 and continuously sparged with argon gas to eliminate oxygen has been performed, and analyzed using an ion chromatograph. The nitrate reduction was found not to exceed 21%. A two-stage process for decreasing the nitrate content of water by reducing nitrates to nitrogen has been reported: photochemical reduction to nitrite; and reduction of nitrite to nitrogen using amidosulphuric (sulfamic) acid.
Spectrophotometric determination of nitrate in samples of river, inland sea and open sea water by photo-induced reduction of nitrate to nitrite in a phosphate-buffered carrier solution at pH 8 flowed through a quartz coil irradiated using a mercury lamp has been studied. The conversion efficiency of nitrate was approximately 50%.
Improved photoreduction efficiency has been achieved by flowing samples in a phosphate-buffer carrier solution having a pH of 7.0 to which the activators ethylenediaminetetraacetic acid (EDTA) or diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) have been added through poly(tetrafluoroethlyene) (PTFE) tubing irradiated with a mercury lamp. The reduction efficiency was found to be 70-84%, with higher conversion efficiencies being obtained using DTPA as the activator.
Total nitrite (reduced nitrate plus any nitrite originally existing in the sample) may be determined by diazotizing with sulfanilamide followed by coupling with N-(1-naphthyl)ethylenediamine dihydrochloride. The resulting water soluble azo dye has a magenta color which may be read at 540 nm, and is known as the Griess test.